Posted this to Debian-KDE list without reply. Hope to get some ideas here.
Posting through IceDove.

Does this mean IceDove works?

Sending failed:
Message sending failed since the following recipients were rejected by
the server:
xxxxxxx@clearmail.com.au (The server responded: "relaying mail to
clearmail.com.au is not allowed")
The message will stay in the 'outbox' folder until you either fix the
problem (e.g. a broken address) or remove the message from the 'outbox'
folder.

Maybe you need additional settings (AUTH, SSL, ...) for the new
provider, but only they can tell you this.

Regards,
Andrei

Icedove does work, and there is nothing extra required for the provider,
we went through that.